Rainfall interception

Water and energy budgets of rain forests along an elevation gradient under maritime tropical conditions

Holwerda, F., 2005. Water and Energy Budgets of Rain Forests Along an Elevational Gradient Under Maritime Tropical Conditions. PhD Thesis, VU University, Amsterdam, The Netherlands.

From the hydrological point of view, mountains present somewhat of a paradox. Although they provide the bulk of the Earth’s freshwater resources, knowledge of the hydrological functioning of mountainous areas is generally much less extensive, reliable, and precise than that of other, often more easily accessible physiographic regions. Indeed, mountain regions have been referred to as ‘the blackest of black boxes in the hydrological cycle’ (Bandyopadhyay et al., 1997). Data collection networks are more difficult to set up and maintain in complex mountainous terrain, particularly in uninhabited forested headwater areas without road access, and minimum recommended instrumental densities are rarely met (Manley and Askew, 1993). Whilst the hydrological knowledge base on mountains in general has increased considerably in the last few decades, most montane research work has focused on determining catchment water and sediment outputs and their distribution in time and space; snow cover and glacier dynamics; or flood frequencies (Molnar, 1990; Lang and Musy, 1990; Bergmann et al., 1991; Young, 1992; Hofer, 1998), as opposed to the underlying hydrological processes (cf. Bonell, 1993). Until very recently (e.g. Motzer, 2003; Schellekens et al., 2004; Goller et al., 2005), the vast majority of this work dealt with mountains in the temperate zone, with very little pertaining to forested tropical mountains (see summaries of early research by Bruijnzeel and Proctor (1995) and Bruijnzeel (2001)). Knowledge of such processes would serve as a basis for increased understanding of how streamflows emanating from tropical mountains might change as a result of changes in climate, including the lifting condensation level, frequency and density of clouds and, by implication, water inputs and evaporative losses (Bruijnzeel, 2001). The average cloud condensation level on tropical islands can be as low as 600-800 m (Malkus, 1955), although on larger mountains situated further inland this may be closer to 2,000 m (Stadtmüller, 1987). Above this condensation level, the hydrology of the forest changes profoundly because of contributions of cloud water (i.e. fog) deposited to the forest canopy (Bruijnzeel, 2001). There is circumstantial evidence that complete conversion of these ‘tropical montane cloud forests’ (TMCF) to pasture or vegetable cropping may have an adverse effect on dry season flows or even on total water yield because of strongly diminished fog interception after clearing (Ingwersen, 1985; Brown et al., 1996). Similar effects may be expected when the average cloud condensation level is raised because of warming of the atmosphere due to global climate change (Still et al., 1999; Foster, 2001), or clearing of forest at lower elevations (Lawton et al., 2001; Van der Molen, 2002).

Hydrometeorology of tropical montane cloud forests: emerging patterns

Bruijnzeel LA, Mulligan M, Scatena FN. 2010. Hydrometeorology of
tropical montane cloud forests: emerging patterns. Hydrological
Processes. DOI: 10.1002/hyp.7974.

altitudinal limits between which TMCF generally occur (800–3500 m.a.s.l. depending on mountain size and distance to coast) their current areal extent is estimated at ¾215 000 km2 or 6Ð6% of all montane tropical forests. Alternatively, on the basis of remotely sensed frequencies of cloud occurrence, fog-affected forest may occupy as much as 2Ð21 Mkm2. Four hydrologically distinct montane forest types may be distinguished, viz. lower montane rain forest below the cloud belt (LMRF), tall lower montane cloud forest (LMCF), upper montane cloud forest (UMCF) of intermediate stature and a group that combines stunted sub-alpine cloud forest (SACF) and ‘elfin’ cloud forest (ECF). Average throughfall to precipitation ratios increase from 0Ð72 š 0Ð07 in LMRF (n D 15) to 0Ð81 š 0Ð11 in LMCF (n D 23), to 1Ð0 š 0Ð27 (n D 18) and 1Ð04 š 0Ð25 (n D 8) in UMCF and SACF–ECF, respectively. Average stemflow fractions increase from LMRF to UMCF and ECF, whereas leaf area index (LAI) and annual evapotranspiration (ET) decrease along the same sequence. Although the data sets for UMCF (n D 3) and ECF (n D 2) are very limited, the ET from UMCF (783 š 112 mm) and ECF (547 š 25 mm) is distinctly lower than that from LMCF (1188 š 239 mm, n D 9) and LMRF (1280 š 72 mm; n D 7). Field-measured annual ‘cloud-water’ interception (CWI) totals determined with the wet-canopy water budget method (WCWB) vary widely between locations and range between 22 and 1990 mm (n D 15). Field measured values also tend to be much larger than modelled amounts of fog interception, particularly at exposed sites. This is thought to reflect a combination of potential model limitations, a mismatch between the scale at which the model was applied (1 ð 1 km) and the scale of the measurements (small plots), as well as the inclusion of near-horizontal wind-driven precipitation in the WCWB-based estimate of CWI. Regional maps of modelled amounts of fog interception across the tropics are presented, showing major spatial variability. Modelled contributions by CWI make up less than 5% of total precipitation in wet areas to more than 75% in low-rainfall areas. Catchment water yields typically increase from LMRF to UMCF and SACF–ECF reflecting concurrent increases in incident precipitation and decreases in evaporative losses. The conversion of LMCF (or LMRF) to pasture likely results in substantial increases in water yield. Changes in water yield after UMCF conversion are probably modest due to trade-offs between concurrent changes in ET and CWI. General circulation model (GCM)-projected rates of climatic drying under SRES greenhouse gas scenarios to the year 2050 are considered to have a profound effect on TMCF hydrological functioning and ecology, although different GCMs produce different and sometimes opposing results. Whilst there have been substantial increases in our understanding of the hydrological processes operating in TMCF, additional research is needed to improve the quantification of occult precipitation inputs (CWI and wind-driven precipitation), and to better understand the hydrological impacts of climate- and land-use change. Copyright  2010 John Wiley & Sons, Ltd.

Throughfall in a Puerto Rican lower montane rain forest: A comparison of sampling strategies

Holwerda, F.; Scatena, F.N.; Bruijnzeel, L.A. 2006. Throughfall in a Puerto Rican lower montane rain forest: A comparison of sampling strategies. Journal of Hydrology 327, :592- 602.

During a one-year period, the variability of throughfall and the standard errors of the means associated with different gauge arrangements were studied in a lower montane rain forest in Puerto Rico. The following gauge arrangements were used: (1) 60 fixed gauges, (2) 30 fixed gauges, and (3) 30 roving gauges. Stemflow was measured on 22 trees of four different species. An ANOVA indicated that mean relative throughfall measured by arrangements 1 (77%), 2 (74%), and 3 (73%) were not significantly different at the 0.05 level. However, the variability of the total throughfall estimate was about half as high for roving gauges (23%) as for fixed gauges (48–49%). The variability of stemflow ranged from 36% to 67% within tree species and was 144% for all sampled trees. Total stemflow was estimated at 4.1% of rainfall, of which palms contributed about 66%. Comparative analysis indicated that while fixed and roving gauge arrangements can give similar mean values, least 100 fixed gauges are required to have an error at the 95% confidence level comparable to that obtained by 30 roving gauges.
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